合肥膨胀土动弹性模量与阻尼比试验研究
Experimental study of dynamic elastic modulus and damping ratio of expansive soil in Hefei
收稿日期: 2019-03-22
Received: 2019-03-22
作者简介 About authors
庄心善(1964—),男,教授,博导,从事环境岩土工程与边坡工程的研究.orcid.org/0000-0001-8319-8726.E-mail:
以合肥膨胀土为研究对象,利用GDS真动三轴仪对土体进行循环动荷载试验,研究不同围压、固结应力比对土体动弹性模量及阻尼比的影响规律.试验结果表明,合肥膨胀土的骨干曲线可以由双曲线描述;在相同条件下,动弹性模量随着围压、固结应力比的增大而增大,随着动应变的增大先急剧减小后趋于平缓;动弹性模量的倒数与动应变呈良好的线性关系,最大动弹性模量随着围压的增大呈线性增大,给出考虑围压影响的最大动弹性模量回归方程;阻尼比随着围压、固结应力比的增大而减小,采用 Darendeli 模型及依据阻尼比与应变的经验关系,得到动模量比衰减模型及阻尼比模型;不同围压、固结应力比下阻尼比与动模量比的关系归一化后可以由修正Hardin-Drnevich公式描述.
关键词:
A series of dynamic triaxial tests were conducted by GDS to analyze the influence of different confining pressures, consolidation ratios on the dynamic modulus and damping ratio of the soil by taking expansive soil in Hefei as the research object. The test results show that the stress-strain backbone curve can be described by hyperbolic function. The dynamic modulus of elasticity increases with the increase of confining pressure and consolidation stress ratio, and decreases sharply at first then tends to be flat with the increase of dynamic strain under the same conditions. The reciprocal of elastic modulus has a linear relationship with dynamic strain. The maximum modulus increases linearly with the increase of confining pressure. A regression model of maximum dynamic modulus and elasticity regression considering confining pressure was developed. Damping ratio decreases with the increase of confining pressure and consolidation stress ratio. An attenuation model of dynamic modulus ratio and a damping ratio model were established based on the Darendeli’s model and empirical relation between damping ratio and strain. The relationship between damping ratio and dynamic modulus ratio under different confining pressure and consolidation stress ratio after normalization processing can be described by the modified formula of Hardin-Drnevich.
Keywords:
本文引用格式
庄心善, 赵汉文, 王俊翔, 黄勇杰.
ZHUANG Xin-shan, ZHAO Han-wen, WANG Jun-xiang, HUANG Yong-jie.
1. 土样制备与试验方案
1.1. 试验仪器和试验土样
试验仪器为英国GDS真动三轴仪,如图1所示,GDS 测量系统可以精准施加围压、轴向压力、反压,并实时记录土样的轴向应变、孔隙压力、体积应变等数据.
图 1
图 1 GDS真动三轴试验仪及电子操控台
Fig.1 GDS dynamic triaxial test device and electronic console
试验土样取自于安徽合肥某高速公路工程. 土样的物理性质指标如表1所示. 表中,ρ为天然密度,W为含水率,W1为液限,Wp为塑限,Gs为比重,Fs为自由膨胀率. 按膨胀土自由膨胀率分级可以归类为弱膨胀土,通过击实试验得出土样最大干密度为1.75 g/cm3,最优含水率为17.5%. 按最优含水率和最大干密度制作重塑土样,试样直径为50 mm,高度为100 mm,制备时分5层捣实,每层进行刮毛处理.
表 1 合肥膨胀土基本物理力学参数
Tab.1
ρ/ (g·cm−3) | W/% | Wl /% | Wp /% | Gs | Fs/% |
1.9 | 21.64 | 72 | 30 | 2.68 | 44 |
1.2. 土样制备与试验方案
先将重塑试样放入饱和器内抽气饱和,再装入GDS真动三轴仪进行反压饱和,直至饱和度B达到 0.95时停止,最后施加设定的围压和轴向压力完成固结. 试验时,循环荷载波形采用正弦波,在不排水条件下分12级施加预设的动应力,每级循环振动 10 次. 当试样轴向应变达到5%时,视为破坏并终止试验[18].
表 2 膨胀土动力加载试验方案
Tab.2
组类 | σ3/kPa | kc | σd/kPa |
1 | 100 | 1.0,1.25,1.50 | 10~120 |
2 | 150 | 1.0,1.25,1.50 | 15~180 |
3 | 200 | 1.0,1.25,1.50 | 20~240 |
2. 试验结果及分析
2.1. 动应力-应变关系
膨胀土试样的动应力σd-应变εd曲线如图2所示.
图 2
图 2 膨胀土动应力-应变关系曲线
Fig.2 Relation curves of dynamic stress-strain of expansive soil
当εd<0.1%时,曲线较陡,随着动应力的增大,应变加速增大,曲线趋于平缓,呈现明显的非线性,整体呈双曲线关系. 在相同条件下,σd-εd曲线随着固结围压、固结应力比的增大而向上显著移动. 当初期εd<0.1%时土体处于弹性变形阶段,土体产生的变形主要为弹性变形,随着动应力幅值的增大,土体逐渐由弹性变形阶段过渡为塑性变形阶段,土体产生的变形以塑性变形为主导,应变发展加快;围压、固结应力比越大,相同应力幅值下土体的动应变越小,说明围压、固结应力比的增大能够抑制膨胀土动应变的发展.
2.2. 动弹性模量特性
动弹性模量Ed取滞回曲线两端点连线的斜率,即
图 3
图 3 不同围压下膨胀土动弹性模量变化曲线
Fig.3 Dynamic elastic modulus-strain curves of expansive soil under different confining pressures
图 4
图 4 不同固结应力比下膨胀土动弹性模量变化曲线
Fig.4 Dynamic elastic modulus-strain curves of expansive soil under different consolidation stress ratios
在相同条件下,Ed 随着围压、固结应力比的增大而增大,表现为 Ed-εd 曲线随着σ3、kc的增大而上移动. 随着围压、固结应力比的增大,土体受到的初始球应力增大,孔隙比变小,土颗粒之间的相互作用增强,相对滑移变得困难,土样抵抗变形的能力提高.
为了便于求最大动弹性模量Ed0,采用Kondner模型[20]:
式中:
图 5
图 5 膨胀土动弹性模量倒数随动应变的变化关系
Fig.5 Relationships between reciprocal of dynamic elastic modulus with dynamic strains for expansive soil
图 6
图 6 不同固结应力比下膨胀土最大动弹性模量随围压的变化关系
Fig.6 Relationships of Ed0 - σ3 for expansive soil under different consolidation stress ratios
图 7
图 7 不同围压下膨胀土最大动弹性模量随固结应力比的变化关系
Fig.7 Relationships of Ed0 - kc for expansive soil under different confining pressures
考虑围压的影响,建立Ed0的回归方程:
式中:pa为大气压强, k 为图6中 σ3 =100 kPa 时Ed0与大气压的比值. 可得与围压有关的指数n为 0.61~0.67,R2=0.99.
2.3. 动模量比衰减模型
将不同试验条件下的Ed/Ed0-εd绘制在半对数坐标中,如图8所示.
图 8
图 8 膨胀土动模量比随动应变的变化关系
Fig.8 Relationships of elastic modulus ratio with dynamic strains for expansive soil
由图8可知,在相同条件下,Ed/Ed0-εd随着围压的增大而向上移动,固结应力比对Ed/Ed0-εd的影响没有明显的倾向性.
式中:
相关系数R2=0.926 1,能够较好地描述合肥膨胀土动弹性模量随动应变的变化规律.
2.4. 阻尼比变化规律
土的阻尼比λ反映了在循环动荷载作用下,部分能量因土体内阻尼而耗散的性质,表现为土动应力-应变滞回圈的滞后性,可以由土体在一个周期内损耗的能量
如图9所示,在σ3=100 kPa,kc=1.00,第10级循环加载的条件下,取试样每级循环荷载中前3次加载的σd-εd试验点拟合出近似椭圆,
图 9
表 3 膨胀土阻尼比的拟合参数
Tab.3
kc | σ3/kPa | n | m | R2 |
1.00 | 100 | 0.242 1 | 1.667 9 | 0.994 9 |
1.00 | 150 | 0.318 8 | 1.636 1 | 0.996 0 |
1.00 | 200 | 0.407 0 | 1.475 8 | 0.996 7 |
1.25 | 100 | 0.208 0 | 1.954 1 | 0.992 1 |
1.25 | 150 | 0.263 6 | 1.763 4 | 0.995 6 |
1.25 | 200 | 0.294 9 | 1.832 2 | 0.997 9 |
1.50 | 100 | 0.160 9 | 2.766 5 | 0.990 1 |
1.50 | 150 | 0.192 9 | 2.472 5 | 0.989 8 |
1.50 | 200 | 0.232 9 | 2.461 0 | 0.992 9 |
图 10
图 10 不同围压下膨胀土阻尼比随动应变的变化关系
Fig.10 Relationships of damping ratio with dynamic strain for expansive soil under different confining pressures
图 11
图 11 不同固结应力比下阻尼比随动应变的变化关系
Fig.11 Relationships of damping ratio with dynamic strain under different consolidation stress ratios
式中:
由图11可知,随着固结应力比的增大,
绘制不同固结应力状态(
图 12
图 12
膨胀土
Fig.12
Relationships between
由图12可知,不同围压、固结应力比下的试验点交错分布在较窄范围内,可以近似归一化为一条曲线. 若采用Hardin-Drnevich公式进行拟合的结果偏差较大,采用修正Hardin-Drnevich公式进行拟合,结果如式(9),R2=0.940 1.
3. 结 论
(1)合肥膨胀土的动弹性模量随着动应变的增大而减小,当动应变较小时,变化幅度较大,随着动应变的增大,变化幅度减小. 随着围压,固结应力比的增大,相同动应变下的动弹性模量增大.
(2)合肥膨胀土的最大动弹性模量随着围压和固结应力比的增大而增大,且与围压呈线性关系,通过分析试验数据得到相关指数为0.61~0.67.
(3) 利用Darendeli 模型描述合肥膨胀土的动模量衰减规律较好. 随着围压的增大,Ed/Ed0-εd曲线向上移动,固结应力比对Ed/Ed0-εd曲线的影响没有明显的倾向性.
(4)在相同条件下,随着围压、固结应力比的增大,膨胀土的阻尼比减小. (
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